Study On The Several Key Scientific Issues Of Laser Shock Processing For Engineering Structure Details Fatigue Parts

Laser shock processing(LSP)is an advanced surface manufacturing technology which uses the high-pulse laser beam irradiating on the surface of the metal coating to gasify and ionize the coating,producing high-energy plasma shock wave to strengthen the metal surface.The application of LSP technology to the details of engineering structures can greatly improve the service life of aviation materials and components and greatly improve the safety and reliability of the system.Many theoretical problems in the application of LSP to the engineering structure details remain to be disclosed,including the propagation and action of the laser shock wave in the engineering structure details;forming mechanism,rule and regulation of residual stress;damage mechanism of components with different material,thickness,geometrical dimension,shape.It is expected to propose a set of theoretical solutions and processing technology through research.In this paper,the existing anti-fatigue technology of LSP applied to the engineering structure details were discussed.The results of domestic and foreign scholars in the field were summarized.The problems and the development of the research and application were discussed,through which related research can be done based on theory,simulation and test methods.Through a large number of simulation and experimental studies,we can clarify the theoretical issues related to the LSP to the engineering structure details;explore the residual stress formation and the action rule,mechanism and regulating method;disclose the characteristics,main characteristic phenomena and the microscopic mechanism existing in the process of LSP;study the external load characteristics of the engineering structure details after LSP to draw the S-N curve of the cyclic loading on the engineering structure details after LSP,which lays a theoretical foundation for the application of LSP technology in engineering structure details and the improvement of the life of key parts and fatigue parts in engineering structures.Main conclusions and innovative achievements are as follows:(1)The propagation of shock wave and its interaction with the engineering structure details were discussed;the formation and distribution of residual stress of engineering structure details under laser-induced shock wave were analyzed;the stress intensity factor differences between hole corner,middle of the hole wall and the fillet structure details after LSP were analyzed.(2)The formation rules and influencing factors of residual stress in hole,fillet and round bar parts were analyzed based on the ABAQUS finite element analysis software and necessary fatigue tests;in order to comprehensively consider the effect of spot overlap rate and the number of impact layer,the concept of single-side impact average rate of coverage was proposed to reveal the dynamic compaction effect on material surface after multiple laser shock;through the study on the peak stress in wall hole and surface stress caused by LSP under different power density(peak pressure).it was revealed that the peak stress increases first and then decreases with the increase of the power density(peak pressure);it was found that the peak stress of hole wall has a certain correspondence with the surface stress which provides an idea and method for indirectly and effectively predicting and controlling the peak stress of hole wall by measuring and controlling the surface stress.(3)Through the theoretical analysis,the power density required for LSP on the hole plate are deduced as (?),and the empirical formula of the power density required for LSP on TC4DT titanium alloy is obtained as (?) by LSP and fatigue test on the hole specimens with different thickness;process parameters selection steps and methods of different materials with different thickness are formulated;TC4-DT titanium alloy sheet and7050-T7451 aluminum alloy sheet as an example were selected and verified by the test.Through the study on the distribution of residual stress and the location and shape of the fatigue source of different thickness specimens,it was found that the fatigue life of laser shock processed specimens with different thickness is obviously different.There is an inevitable connection between the effect of LSP and thickness of specimens,so the concept of the thickness effect of hole specimens after LSP is proposed.(4)Through a large number of simulations and experiments,the distribution of residual stress in models with different surface shapes and curvatures after shock has been studied.The results show that the distribution of residual stress is different in the bending direction of the model and concave,flat and convex model have different depth of influence layer;it has been proved by experiment that the LSP with optimized parameters can effectively improve the fatigue life of fillet structure.The results also showed that for the round bar sample,the laser shock caused the inward shift of stress position and the increase of peak tensile stress.The titanium alloy round bar is not conducive to the pull-pull fatigue performance after laser shock,which showed a negative gain;the aluminum alloy round bar sample which was exerted less energy shock showed a better gain;under the bending load,the fatigue life of the titanium alloy rod increased greatly after the laser shock,so it is considered that the choice of service environment after LSP is an important issue.(5)The fatigue test and statistical analysis of 7050T7451 aluminum alloy,TC4-DT,TC21titanium alloy material,and different engineering structure details(holes,fillets,bars,etc.)with different thickness were carried out under different stress levels;effect of LSP on different materials,geometric size and shape and the effects of process on the fatigue resistance of the engineering structure detail fatigue structure were studied and discussed;referring to the HB/Z 112-1986?method of statistical analysis of the material fatigue specimen',according to the method of paired comparison analysis,under the 95%confidence level,the S-N curve in each situation were drawn and studied through the statistical analysis of the test results.(6)Through the study on fatigue life and fracture characteristics under different experimental parameters,the mechanism and rule of fatigue source transferring and its relationship with fatigue with the increase of laser shock energy life have been found;the rule and mechanism of fatigue source initiation and propagation of crack under different external stress level was also revealed;the stress distribution in fillet and round bar specimen and the morphological characteristics and rule of fatigue fracture under different cyclic loads were investigated.